Add bit about wrap_fn to middlewares page (#104)

content/docs/middleware.md

@@ -29,6 +29,10 @@ The following demonstrates creating a simple middleware:
 
 {{< include-example example="middleware" file="main.rs" section="simple" >}}
 
+Alternatively, for simple use cases, you can use [*wrap_fn*][wrap_fn] to create small, ad-hoc middlewares:
+
+{{< include-example example="middleware" file="wrap_fn.rs" section="wrap-fn" >}}
+
 > Actix-web provides several useful middlewares, such as *logging*, *user sessions*,
 > *compress*, etc.
 
@@ -130,3 +134,4 @@ into a response.
 [envlogger]: https://docs.rs/env_logger/*/env_logger/
 [servicetrait]: https://docs.rs/actix-web/1.0.2/actix_web/dev/trait.Service.html
 [transformtrait]: https://docs.rs/actix-web/1.0.2/actix_web/dev/trait.Transform.html
+[wrap_fn]: https://docs.rs/actix-web/1.0.5/actix_web/struct.App.html#method.wrap_fn

examples/middleware/src/main.rs

@@ -2,6 +2,7 @@ pub mod default_headers;
 pub mod errorhandler;
 pub mod logger;
 pub mod user_sessions;
+pub mod wrap_fn;
 
 // <simple>
 use actix_service::{Service, Transform};

examples/middleware/src/wrap_fn.rs

@@ -0,0 +1,20 @@
+// <wrap-fn>
+use actix_service::Service;
+use actix_web::{web, App};
+use futures::future::Future;
+
+fn main() {
+    let app = App::new()
+        .wrap_fn(|req, srv| {
+            println!("Hi from start. You requested: {}", req.path());
+            srv.call(req).map(|res| {
+                println!("Hi from response");
+                res
+            })
+        })
+        .route(
+            "/index.html",
+            web::get().to(|| "Hello, middleware!"),
+        );
+}
+// </wrap-fn>